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Leonotis leonurus (L. leonurus) and Artemisia afra (A. afra) are two of the most commonly used medicinal plants in South Africa traditionally advocated for use in asthma. However, proper scientific studies to validate these claimed uses are lacking and little is known about the mechanisms for this effect. These plants contain flavonoids, which are reported to have smooth muscle relaxant activity and may be responsible for the activity of these two plants. The objectives of this study were to: (1) determine and compare the flavonoid profiles and levels in A. afra and L. leonurus, (2) compare the respiratory smooth muscle relaxant effects
of freeze-dried aqueous extracts of A. afra and L. leonurus and (3) investigate whether K⁺ - channel activation (i.e. KATP channel) is one possible mechanism of action that can explain the effect obtained in traditional use of these two plants. It was hypothesized that: (1) the flavonoid levels and profile of A. afra would be greater than the flavonoid levels and profile of L. leonurus, (2) A. afra would have a more potent respiratory muscle relaxant effect than L. leonurus and (3) A. afra and L. leonurus will inhibit K⁺ - induced contractions in a superior manner than carbachol and histamine - induced contractions. To realize these objectives, freeze-dried aqueous extracts (FDAE) of the dried leaves of the two plants were prepared. A validated HPLC assay was developed and used to identify and
determine the levels of luteolin in the plant preparations. Solutions of the plant extracts were studied in the isolated guinea-pig trachea tissue preparation in the presence of carbachol, histamine and KCL. The possible mechanism of action of the two plants was determined by cumulative log dose-response curves (LDRC) for carbachol, histamine and KCL in the absence and presence of 1, 30 and 100 mg/ml solutions of the plant extracts. The flavonoid profile of un-hydrolyzed and hydrolyzed L. leonurus was greater than that of un-hydrolyzed and hydrolyzed A. afra. The levels of free and total luteolin in A. afra FDAE (8.977 ± 0.73 μg/ml and 16.394 ± 0.884 μg/ml, respectively) were significantly (p < 0.001) higher than that in L. leonurus FDAE (0.929 ± 0.066 μg/ml and 3.093 ± 0.531 μg/ml, respectively). L. leonurus and A. afra relaxed tracheal smooth muscles contracted with histamine, KCL and carbachol in a dose dependent manner. The degree of relaxant activity of
L. leonurus versus the three inducers of contraction (agonists) could be classified as KCL > carbachol > histamine, with EC₅₀ values of 9.87, 29.34 and 94.76 mg/ml, respectively. The A. afra tracheal smooth muscle relaxant activity was categorized as carbachol > histamine > KCL, with EC₅₀ values of 13.93, 15.47 and 19.88 mg/ml, respectively. Overall, A. afra which contained the higher levels of luteolin, was more potent at relaxing the guinea pig tracheal smooth muscle than L. leonurus. Collectively, the results confirm that aqueous solutions of A. afra and L. leonurus as used in local traditional practice have potent but different degrees of bronchodilator activities that could be useful in the treatment of asthma, and that these actions may be related to each plant's luteolin (or flavonoid) levels. Moreover it is very unlikely that KATP channels are primarily responsible for the actions of A. afra and L. leonurus, but rather that more than one mechanism of action is involved in the tracheal smooth muscle relaxant effects of these two plants.